Tuneable magnetic properties of nanoporous metal electrodes via electrochemical reactions

Owed to their high surface-to-volume ratios, nanoporous metals offer the possibility to adjust intrinsic bulk properties, such as electrical conductivity or magnetic susceptibility, via surface modulations. For this purpose, electrochemical reactions on nanoporous electrodes are particularly suitable tools to achieve surface states with controlled chemical composition. Magnetic properties of metals and alloys are highly sensitive to both chemical environment and electronic structure, which serves as basis for all our investigations.

Nanoporous metal and alloy electrodes were fabricated using the method of electrochemical dealloying, a selective dissolution process from binary or ternary alloys. By exposing the precursor to an etching agent, the least noble component of the alloy is gradually removed, while enhancing the surface diffusivity of the more noble compound(s) [1]. Appropriate processing conditions enable the formation of nanoporous structures, with the main benefit being adjustable pore sizes. Nanoporous palladium (npPd) and nanoporous copper-nickel alloys (npCuNi) were used as model systems, as both systems are close to the limit of ferromagnetic order, leading to a strong response of magnetic moment upon surface treatment.

Magnetic tuning experiments were performed in an electrochemical cell, designed for special use in a SQUID (Superconducting Quantum Interference Device) magnetometer. This approach combines highly sensitive measurements of the magnetic moment in a SQUID with the complete toolbox of electrochemical techniques, in order to bias the sample surface in-situ during magnetic measurements [2]. Electrochemical hydrogen adsorption and oxide formation processes were studied in detail and could be directly related to changes in magnetic behaviour.

Alterations of the magnetic moment of more than 20% were measured for npCuNi upon surface oxidation in aqueous electrolyte, which will be discussed in terms of a perturbation of the magnetic order of the nickel atoms.

This work is financially supported by the Austrian Science Fund (FWF): P30070-N36.

[1] I. McCue et al., Ann. Rev. Mater. Res. 46 (2016) 263
[2] E.-M. Steyskal et al., Beilst. J. Nanotech. 4 (2013) 394